Field of the Invention
This invention relates to apparatus for exposing or imaging an IC (integrated circuit) circuit pattern on a wafer or photo mask coated with a photosensitive material and more particularly to a system for controlling the exposure value in the apparatus.
In recent years, there has been a demand for ever increasing fineness of circuit patterns in the manufacture of semiconductor devices, particularly ICs and also a high degree of exposure control has been required for apparatus designed to print or place a pattern having a line-width of 1 .mu.m or less on a wafer or so-called exposure apparatus.
Generally, the exposure apparatus for exposing an image of a circuit pattern on a wafer or photo mask should preferably always apply a constant exposure value to the photosensitive material (photo-resist) applied onto a wafer.
The exposure apparatus projects an image of a reticle or photo mask having an IC circuit pattern formed thereon onto a photosensitive material through an optical system including an exposure radiation source, e.g., a light source, lenses, etc. Where the exposure light source is always turned on, the exposure apparatus must include a shutter for controlling the exposure time of the photosensitive material. Moreover, where the light intensity of the exposure light source is subject to variation due to deterioration of the exposure light source, the exposure apparatus must include means for measuring the exposure value applied to the photosensitive material by the light from the exposure light source through the shutter and means for controlling the shutter in accordance with the measured exposure value for the purpose of maintaining the exposure value of the photo-sensitive material constant. The light measuring means and the shutter control means may be conceived to operate as follows. The control means changes the shutter from the shut position (closed position) to the wide open position and the light measuring means measures the amount of the exposure light in response to the change. When the exposure value reaches a proper exposure value, the shutter is changed again from the wide open position to the shut position.
However, there is a disadvantage that the photosensitive material is still exposed even during the transition of the shutter from the wide open position to the shut position in response to the attainment of the proper exposure value by the exposure value of the photosensitive material.
While the excessive exposure value to the photosensitive material can be estimated to determine the total exposure value if the light intensity of the light source is always constant, in fact, however, the brightness of the light source varies considerably due to the deterioration of the lamp. This variation (E.sub.o -E.sub.o ') of the excessive exposure value can be ignored if the light intensity of the light source is low and the exposure time is sufficiently long. However, if the light intensity of the light source increases thus increasing the ratio of the exposure value during the shutting operation of the shutter to the total exposure value, the variation of the total exposure value due to the variation of the excessive exposure value presents a serious problem. While it is generally considered that the variation of the total exposure value must be less than several percent in the exposure value control, in the above-mentioned case the conventional apparatus gives no consideration to the excessive exposure value with the resulting disadvantage that it is impossible to obtain a stable exposure value over a long period of time.